"I was impressed," says Harvey S. Wiener (at the time a reluctant athlete fighting a mid-30s' paunch). He had just read the first three chapters of James Fixx's super best-seller that fueled the nation's running craze.

"'He's right,' I told myself."

But: "He's got the wrong sport."

Wiener, an English professor at City College of New York, had only recently taken up swimming to strip away unwanted pounds. As he built up his distance from a couple of laps to mile-long workouts (72 pool lengths) a year later, he felt the kind of emotional charge Fixx wrote about.

"I discovered not only great benefits in terms of my physical health," says Wiener, but also -- and this is most important -- a kind of inner strength and calm which I did not expect at all. Swimming, I learned, was a sport to develop minds as well as muscles."

Memorial Day weekend traditionally inaugurates the summer swimming season with the opening of outdoor pools.But seasonal splashers can learn from the growing numbers of enthusiasts -- who regularly take to the (heated) water even in the coldest months. Not only is it fun, it's good for you.

"Swimming is the closest thing on this earth to a perfect sport," says professor Jane Katz, a former Olympic competitor who teaches aquatics at Bronx Community College of the City University of New York. "It exercises all the major muscles of the body; it's the inexpensive, fun, social, graceful, sensual, safe, gentle way to achieve fitness."

Water "feels good and is good -- for your body and your soul. Just being in it is relaxing and exhilarating at the same time."

Although running and jogging beat out swimming -- barely -- as the most popular exercise here (according to a Washington Post survey), Katz warns:

"Did you know, for example, that every time a runner's foot hits the ground, his body receives a shock of up to three times his or her body weight? More and more people are discovering that thier feet, leg muscles, knees and hip joints simply cannot stand up over the years to that kind of constant pounding."

In water, "There's no jarring against any hard surface: There's only the smooth, rhythmic motion. . . . That's why swimming is the sport of the future." The benefits of swimming a half mile, she notes, "are comparable to those of running two miles."

Since swimming "uses the arms and upper body more than the legs," she suggests adding running, walking, cycling, skiing and other nonaquatic sports to an exercise program.

Wiener and Katz have each completed a book -- based on the Fixx model -- on the benefits of lap-swimming, along with tips of getting started and improving strokes. That they tend to complement rather than duplicate -- he's strong on the joy of the sport; she details the aerobic route from tadpole to "super" swimmer -- may be a result of their childhoods.

Wiener, 41, grew up as something of a "neighborhood klutz" at sports, graduating from college at a hefty 214 pounds on a 6-foot-2 frame. Only in the water did he feel like he had any athletic ability. Later, when tension on a temporary job led to overeating, "I turned by instinct to swimming because it was the only exercise that didn't merely at the thought of it cover me with a wave of nausea."

In Total Swimming (Fireside, 368 pages, $6.95 paper), he says now (at 178 pounds), "I'd sooner not eat than not swim. It's the part of my life that makes the rest of my life run smoothly." One thing he decidely does not do is compete, preferring the "routine" of regular exercise to the "regimen" of racing. To compete, he believes, is to replace work stress with play stress.

Katz, exceptionally trim at 38, was a member of the 1964 synchronized swimming team at Tokyo. She continues to train vigorously (several miles a week) and races in the Masters series for adults from 25 up to their 70s and 80s, holding a number of masters records in her age category. "The one-mile is my best event."

In Swimming for Total Fitness (Doubleday Dolphin, 363 pages, $10.95 paper), Katz explains why she loves to race: " . . . the challenge to test your abilities to the utmost, and then to improve on them. You'll find it an amazing high to swim competitively." Lap-swimmers can become racers, she says, the way beginning runners progress to marathons.

Whatever level of ability suits you, you will find company in Washington's pool morning, noon and night.

A dedicated group of mostly middle-age regulars shows up weekdays for the before-breakfast Sunrise Swim from 6:30 to 9 a.m. at the Jewish Community Center in Rockville. At the downtown YMCA, dozens of office workers of both sexes skip lunch to get in a 45-minute mile. ("That's their liquid lunch," says Katz.) And after 5, they jam the lanes for a tension-reducing workout.

Admittedly, finding a place to swim is not easy as stepping outside your front door and jogging around the block. In the summer, community pools often are clogged with youngsters, making your laps a game of dodge 'ems unless time periods are set aside for adults. Health-club pools are a less-crowded alternative, but they can be costly.

But as one regular at the downtown Y points out, the expense is a matter of priorities. A full Y membership costs aobut $31 a month after an intial fee to join. When a non-swimming colleague challenged him for paying so much, he retorted, "You probably spend that much on cigarettes."

Adds Katz: "It's cheaper than hypertension pills for nervousness."

In the heat of the summer, most people are eager to plunge into a cool pool, or lake. But when the winter winds howl and slush blankets the pavement, it takes a certain self-discipline to bundle up in coat, hat, gloves, scarf and trudge off for a swim at the Y. You get, say Wiener and Katz, used to it.

And, they add, forget about catching a cold afterward. It probably won't happen if you dress sensibly.

To the racquetball nut, the thought of stroking back and forth in a chlorine fog for a half-hour or more may sound as exciting as a drive to Rockville. Wiener disagrees.

"Dull? I don't find it boring at all. Something quite magical happens. I get some of my best ideas -- a sudden flash of insight. I'm transported mentally. If I'm not watching, I could swim on forever."

To keep up your interest, Katz suggests alternating competitive strokes -- crawl, backstroke, breaststroke, butterfly -- with recreational and resting strokes, such as sidestroke and treading. Vary the pace. Swim "against the clock or your own internal pace."

Racers have been using a variation of the crawl -- the "S"-pattern arm pull -- which, she says, is becoming popular with non-competitors -- because it's "more efficient" and "more powerful" than the straight-arm pull. The arms make a wider sweep outward, then tuck in at the waist before thrusting out again in an "S" movement.

The "ideal" workout, she says, is 30 minutes three times a week. An experienced swimmer such as Wiener can comfortably cover three-quarters of a mile or more in that time.

For increased fitness, Katz recommends going beyond straight lap-swimming. Practice technique-improving drills that "progressively push your body a little harder and a little further than is comfortable."

One reason many lap-swimmers tire easily, she says, is because they don't know how to breathe properly. "Most people don't do it well. They go two fast lanes and then poop out. Breathing is basic."

The key is to exhale continuously -- while your face is in the water -- through both the nose and the mouth. Inhale deeply through both when your face emerges.

Katz recommends five minutes of warm-up exercises before you jump in the pool -- or begin with slow laps -- and several "easy, relaxed" cool-down laps at the end.

"Don't just pop out of the pool after a vigorous swim. It's the worst thing to do. You want to get your body back to the state where it was before you got in."

And don't splash so much.

"Most people overkick with a lot of splashing. They don't go anywhere."